1. Technical Field
The present invention relates to an ink set, particularly to an ink set that can form an image with excellent ozonefastness, lightfastness and moisturefastness, as well as to an ink cartridge, inkjet recording method, and recorded matter that use this ink set.
2. Related Art
In recent years attention has focused on the inkjet recording method. The inkjet recording method is a printing method where printing is performed by spraying an ink composition as small droplets, and these small droplets adhere to a recording medium such as paper or the like. This method has the characteristics of enabling high-speed printing of a high-resolution high quality image using a relatively inexpensive device. Furthermore, inkjet recording devices that use this method have wide acceptance in the market because of the excellent printing quality, low cost, relatively quiet operation, and excellent graphics capability. Of these devices, thermal (bubblejet (registered trademark)) and piezoelectric drop on demand printers have been especially commercially successful, and are widely used as printers for personal computers at the office and home.
Furthermore, in recent years, printed matter has been obtained by forming a color image using an inkjet recording method that uses a plurality of color ink compositions. Generally, formation of a color image is performed using three colors, namely a yellow ink composition, a magenta ink composition, and a cyan ink composition, or if desired, using four colors by adding a black ink composition. Furthermore, color images are also formed using six colors with the addition of a light cyan ink composition and a light magenta ink composition to the aforementioned four colors, or using seven colors by also adding a dark yellow ink composition. Combinations of two or more of these ink compositions are referred to as an ink set.
The ink compositions which are used in forming the aforementioned color image are required to have good coloring properties for the individual color of each ink composition, as well as the capability to create favorable intermediate colors when a plurality of ink compositions are combined, and the recorded matter obtained thereby must not discolor when stored.
Furthermore, in recent years continuous improvements have been made in the head, in the composition, recording method, and media in order to achieve “picture quality” printing using a color inkjet printer, and the image quality obtained is similar to a photograph and at a level indistinguishable from a “silver chloride photograph”. On the other hand, efforts are underway to improve the storage properties of the images of recorded matter obtained using a color inkjet printer by improving the ink compositions and the media, and in particular the lightfastness of the image has been improved to a practical level (for example, refer to JP-A-2000-290559 and JP-A-2001-288392). However, degradation of the image occurs not only because of the effect of light, but also because of oxidation due to oxidative gases existing in the environment such as ozone, and water (humidity) in the environment, and therefore discoloration and fading of the image will proceed. Therefore, in order to further improve the storage properties of the image of the recorded matter, the ozonefastness and humidityfastness should also be improved along with improving the lightfastness of the image formed by the ink composition. Therefore, various investigations have been performed in order to improve the lightfastness, ozonefastness, and humidityfastness of the inkjet recorded matter, but in particular, efforts to date have been made to improve the ozonefastness of the colorants used in the ink composition (refer to International Patent WO02/060994 and JP-A-2002-371214).
Furthermore, in an ink set, if the lightfastness, ozonefastness, and humidityfastness of a specific ink composition are noticeably inferior to the lightfastness, ozonefastness, and humidityfastness of the other ink compositions, the color formed by that specific ink composition will fade and discolor faster than the other colors, so the color balance of the entire image will be poor, and an observer will be aware of the degradation in the picture quality of the image in a shorter period of time than they could be aware of fading of an image formed by only one ink composition. Therefore, with an ink set, in addition to improving the lightfastness, ozonefastness, and humidityfastness of each of the ink compositions which form the ink set, preferably the level of the lightfastness, ozonefastness, and humidityfastness of each of the ink compositions, or in other words the degradation speeds and the fading speeds of an image formed by each of the ink compositions due to light, ozone, and water will preferably be as similar as possible. In other words, the lightfastness, ozonefastness, and humidityfastness of each of the ink compositions that make up the ink set should be excellent, and preferably the difference in the lightfastness, ozonefastness, and humidityfastness of each ink composition will be small.
Furthermore, images with varying color intensities can be formed and an image without a grainy feel can be obtained by using an inkjet that includes two different tints of ink compositions with different color concentrations in the same color system. An ink set that includes two types of ink compositions with different color concentrations in this manner is used primarily for printing photograph images, but when forming the photographic image, generally an ink composition with a low color concentration is usually used in order to relieve or eliminate the graininess of the image. Furthermore, when evaluating the lightfastness, ozonefastness, and humidityfastness, a pattern with an optical concentration near 1.0 is formed as an evaluation sample using ink compositions with low color concentration. Therefore, in order to improve the lightfastness, ozonefastness, and humidityfastness of the overall ink set and the lightfastness, ozonefastness, and humidityfastness of recorded matter with photographic images, improving the lightfastness, ozonefastness, and humidityfastness of the ink compositions with low color concentration is critical. On the other hand, ink compositions with rich color concentrations are used for printing extremely vivid images and graphic art patterns, so improving the lightfastness, ozonefastness, and humidityfastness of these ink compositions is also important.
Furthermore, a black ink composition plays an important role from the viewpoint of achieving image contrast in the image, and therefore black ink compositions are commonly included in an ink set. Therefore, when a black ink composition is included in the ink set, the black ink composition must have excellent lightfastness, ozonefastness, and humidityfastness, while at the same time the degradation speed of the black ink composition due to light, ozone, or water must not differ extremely from the degradation speed of the other ink compositions which make up the ink set.
As described above, in an ink set, each of the individual ink compositions that make up the ink set must have good lightfastness, ozonefastness, and humidityfastness, while at the same time the balance between the lightfastness, ozonefastness, and humidityfastness of the different ink compositions must be good, and when exposed to light, ozone, and water, a specific color must not fade and/or discolor faster than the other colors. Furthermore, a bronzing phenomenon may occur when attempting to improve the lightfastness, ozonefastness, and humidityfastness of each of the ink compositions which make up an ink set, and this bronzing phenomenon must also be reduced. Bronzing phenomenon refers to the phenomenon of floating red visible ink printed regions when printing is performed at high duty, and is thought to have a tendency to easily occur particularly when fill printing (printing to fill in an area at 100% duty) using an ink composition that uses a metal phthalocyanine based dye as the colorant. The bronzing phenomenon affects the color balance of the entire image, and can lead to poor image quality.
Furthermore, any ink set that improves the degradation of image quality or the like and that is capable of recording and image with good ozonefastness on a recording medium, as well as an ink cartridge that stores this ink set, a recording method that uses this ink set, and recorded matter that is recorded using this ink set has previously been proposed (for example, refer to JP-A-2005-105135).
While these ink sets or the like provide a practical level of lightfastness and ozonefastness for consumer applications, but there is demand for further improvement of lightfastness and ozonefastness for diversified applications and for versatility at the edge of inkjet technology. Furthermore, ensuring humidityfastness is an important characteristic for accommodating diversity of applications.